This invention generally relates to ultrasonic transducers comprising piezoelectric elements sandwiched between backing/matching layers. In particular, the invention relates to a method for constructing ultrasonic transducers having an improved bandwidth.
Conventional ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements sandwiched between a pair of backing/matching layers. Such piezoelectric elements are constructed in the shape of plates or rectangular beams bonded to the backing and matching layers. The piezoelectric material is typically lead zirconate titanate (PZT), polyvinylidene difluoride (PVDF), or PZT ceramic/polymer composite.
Almost all conventional transducers use some variation of the geometry shown in FIG. 1. The basic ultrasonic transducer 2 consists of layers of materials, at least one of which is a piezoelectric plate 4 coupled to a pair of electric terminals 6 and 8. The electric terminals are connected to an electrical source having an impedance Zs. When a voltage waveform v(t) is developed across the terminals, the material of the piezoelectric element compresses at a frequency corresponding to that of the applied voltage, thereby emitting an ultrasonic wave into the media to which the piezoelectric element is coupled. Conversely, when an ultrasonic wave impinges on the material of the piezoelectric element, the latter produces a corresponding voltage across its terminals and the associated electrical load component of the electrical source.
Typically, the front surface of piezoelectric element 4 is covered with one or more acoustic matching layers or windows (e.g., 12 and 14) that improve the coupling with the media 16 in which the emitted ultrasonic waves will propagate. In addition, a backing layer 10 is coupled to the rear surface of piezoelectric element 4 to absorb ultrasonic waves that emerge from the back side of the element so that they will not be partially reflected and interfere with the ultrasonic waves propagating in the forward direction. A number of such ultrasonic transducer constructions are disclosed in U.S. Pat. Nos. 4,217,684, 4,425,525, 4,441,503, 4,470,305 and 4,569,231, all of which are commonly assigned to the instant assignee.
The basic principle of operation of such conventional transducers is that the piezoelectric element radiates respective ultrasonic waves of identical shape but reverse polarity from its back surface 18 and front surface 20. These waves are indicated in FIG. 1 by the functions Pb(t) and Pf(t) for the back and front surfaces respectively. A transducer is said to be halfwave resonant when the two waves constructively interfere at the front face 20, i.e., the thickness of the piezoelectric plate equals one-half of the ultrasonic wavelength. The half-wave frequency fo is the practical band center of most transducers. At frequencies lower than the half-wave resonance, the two waves interfere destructively so that there is progressively less and less acoustic response as the frequency approaches zero. Conversely, for frequencies above the half-wave resonance there are successive destructive interferences at 2fo and every subsequent even multiple of f0. Also, there are constructive interferences at every frequency which is an odd multiple of f0. The full dynamics of the transducer of FIG. 1 involve taking into account the impedances of each layer and the subsequent reflection and transmission coefficients. The dynamics of the transducer are tuned by adjusting the thicknesses and impedances of the layers.
The conventional piezoelectric element has very thin boundaries and launches waves of opposite polarity from front and back faces, as shown in FIG. 2A. Very wide bandwidth signals have been shown so that operation of the transducer can be examined using impulse response concepts. These waves are indicated in FIG. 2A by the functions xe2x80x94P(txe2x80x94T) and P(t) for the back and front surfaces respectively, where T is the transit time across the piezoelectric element 4. The waves are shown after they have propagated some distance. (For the sake of clarity two negatively propagating waves have been suppressed from FIG. 2A.) The destructive resonance at 2f0 is a fundamental limitation of these conventional piezoelectric elements.
The present invention is an ultrasonic transducer which overcomes the destructive interference inherent in all transducers (plate and beam) comprising piezoelectric elements sandwiched between backing/matching layers. The basic principle of the invention is to cause the wave emanating from the back surface of the piezoelectric element to spread over time as if passed through a low-pass filter, while the wave emanating from the front surface remains unaltered. The combination of the two waves, at frequencies which would produce destructive interference in a conventional transducer, produces no destructive interference in an ultrasonic transducer in accordance with the invention.
The foregoing effect can be achieved in accordance with a first preferred embodiment of the invention by altering the texture of the transducer back surface. In particular, a roughened back surface is used to excite a distributed ultrasonic waveform, which is spread over time relative to the sharply defined waveform excited at the front surface. The back surface can be roughened, for example, by chemical etching or by knurling or cutting the surface with a diamond saw. This roughening of the back surface has the effect of lowpass filtering the wave emanating from the back surface and subsequently reducing its magnitude.
In accordance with a second preferred embodiment of the invention, an ultrasonic transducer is made having a spatially graded piezoelectric coupling. The piezoelectric coupling is varied in a manner that produces a low-pass filtering operation for only one of the two ultrasonic wave sources. In particular, the piezoelectric coupling has a spatial distribution that rises smoothly from zero at the back face, reaches a plateau and drops abruptly at the front face.
A spatial distribution of the piezoelectric coupling along the width of the piezoelectric element can be achieved by partially de-poling the piezoelectric material, e.g., by heating the back side of the piezoelectric element to a temperature above the Curie temperature while maintaining the front side of the element cold.
Ultrasonic transducers having a broadband transfer function can be produced using either of the preferred methods of manufacture. In contrast to conventional ultrasonic transducers wherein destructive interference results in fractional bandwidths of approximately 70%, incorporation of the invention in an ultrasonic transducer prevents destructive interference, thereby permitting arbitrary bandwidth.
Applying the teaching of this invention to the field of medical diagnostic imaging, multiband transducers can be readily designed with superior bandwidths. Also, very broadband signals may be used, which provides enhanced image quality.